1 Field of the Invention
The present invention relates to a wire-bonding method for bonding the first and second bonding points of a workpiece and more particularly to a method for reducing a wire loop curvature.
2. Prior Art
A wire-bonding is one of the processes for manufacturing an integrated circuit.
As shown in FIG. 2, a wire 3 is connected between a pad 1a (a first bonding point) of a pellet 1 and a lead 2a (a second bonding point) of a lead frame 2 by wire-bonding.
Several wire-bonding methods have been developed and proposed, and one of the most general methods of wire-bonding will be described below with reference to FIGS. 3(a) through 3(g).
First, as shown in FIG. 3(a), a ball 3a is formed at a tip end of a wire 3 extended through a capillary 4 by a spark discharging from an electric torch 5, and the electric torch 5 is moved in the direction indicated by an arrow after the spark discharge.
Next, as shown in FIGS. 3(b) and (c), the capillary 4 is moved and located above a first bonding point 1a, then the capillary 4 is moved downwardly, and the ball 3a is connected to the first bonding point 1a.
After the ball 3a is connected to the first bonding point 1a, the wire 3 is fed out of the tip end of the capillary 4 while the capillary 4 is moved upwardly as shown in FIG. 3(d), and the capillary 4 is located above a second bonding point 2a as shown in FIG. 3(e).
Afterwards, the capillary 4 is moved toward the second bonding point 2a, and the wire 3 is fastened thereto as shown in FIG. 3(f). After the wire 3 is connected to the second bonding point 2a, the capillary 4 is moved toward and located at a prescribed position. A clamp 6 is closed to hold the wire 3, and the wire 3 is cut when the capillary 4 and the clamp 6 are moved upwardly as shown in FIG. 3(g). As a result, the wire 3 is bonded between the first and second bonding points. One process of the wire-bonding is thus completed.
Wire-bonding methods of this kind are described in, for example, the Japanese Patent Application Laid-Open No. 57-87143 and the Japanese Patent Application Publication No. 1-26531.
However, in recent years, gaps between the adjacent wires needs to be narrower in view of downsizing of an integrated circuit. As a result, there is a probability of a short circuit between the wires 3, so that an allowance of a wire loop curvature becomes severe.
FIG. 4 is a plan view of the first and second bonding points 1a and 2a. A straight line 3 which extends between the first bonding point 1a and the second bonding point 2a is an intended line of a wire. In the specification, a "wire loop curvature" means that a wire is bent and not in accord with the line 3, for example, as shown by a line 3'.
In the wire-bonding method described above, when the capillary 4 is positioned above the second bonding point 2a at which a distance between the surface of the second bonding point and the lower end of the capillary 4 is very short, the wire 3 is supplied as shown in FIG. 5(a). In this condition, the capillary 4 is moved downwardly to connect the wire 3 onto the second wire bonding point 2a as shown in FIG. 5(b). Therefore, an excessive wire, which is a part of the wire 3 having the length "C", may cause the wire 3 to be curved when the capillary 4 is pressed against the second bonding point.
If the excessive wire C causes the wire 3 to push upwardly, there is no problem. However, when the capillary 4 is pressed against the second bonding point, if the lower portion 3b of the excessive wire comes into contact with the lead frame 2 and the excessive wire urges the wire to move toward the first bonding point 1a, the wire 3 is yielded as shown by the line 3' in FIG. 4.